Exposing the skin of mice to UV radiation interferes with the induction of delayed and contact hypersensitivity immune responses initiated at nonirradiated sites. The identity of the molecular target in the skin for these Immunosuppressive effects of UV radiation remains controversial. To test the hypothesis that DNA is the target for UV-induced systemic immunosuppression, we exposed C3H mice to UV radiation and then used liposomes to deliver a dimer-specific excision repair enzyme into the epidermis in situ. The application of T4 endonuclease V enulated in liposomes to UV-irradiated mouse skin decreased the number of cyclobutane pyrimidine dimers in the epidermis and prevented suppression of both delayed and contact hypersensitivity responses. Moreover, the formation of suppressor lymphoid cells was inhibited. Control, heat-inactivated endonuclease encapsulated in liposomes had no effect. These studies demonstrate that DNA is the major target of UV radiation in the generation of systemic immunosuppression and suggest that the primary molecular event mediating these types of immunosuppression by UV radiation is the formation of pyrimidine dimers. Furthermore, they illustrate that the delivery of lesion-specific DNA repair enzymes to living skin after UV Irradiation is an effective tool for restoring Immune function and suggest that this approach may be broadly applicable to preventing other alterations caused by DNA damage.Wavelengths of UV radiation in the middle, or UV-B (280-320 nm), range can impair a variety of immune responses in humans and laboratory animals both locally, within UVirradiated skin, and systemically, at distant sites (1). Exposure of mice to UV-B radiation interferes with the rejection of UV-induced skin cancers and the induction of delayed and contact hypersensitivity (DHS and CHS) responses initiated at unirradiated sites; these forms of immune suppression are associated with the induction of antigen-specific suppressor T lymphocytes (2). How UV-B radiation exerts its systemic, immunosuppressive effects is a question of considerable interest, both for understanding the regulatory pathways governing these immune responses and for assessing the potential effects of UV-B radiation on human health. The DHS response is particularly important in this regard because this T-lymphocyte-mediated immune reaction is responsible for protection against many chronic infectious diseases.Current experimental evidence implicates soluble substances derived from UV-irradiated keratinocytes as the probable mediators of UV-induced systemic suppression of DHS and CHS responses (3-5). However, the initial photobiological reaction responsible for triggering the cascade of events leading to activation of the suppressor pathway of the immune response remains controversial. Based on an in vivo action spectrum for systemic suppression of CHS in the mouse, it has been proposed that urocanic acid, a deamination product of histidine, present in the stratum corneum, is the photoreceptor for this form of UV-induce...

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UVB-induced DNA damage is a crucial event in UVB-mediated apoptosis. On the other hand, UVB directly activates death receptors on the cell surface including CD95, implying that UVB-induced apoptosis can be initiated at the cell membrane through death receptor clustering. This study was performed to measure the relative contribution of nuclear and membrane effects in UVB-induced apoptosis of the human epithelial cell line HeLa. UVB-mediated DNA damage can be reduced by treating cells with liposomes containing the repair enzyme photolyase followed by exposure to photoreactivating light. Addition of photolyase followed by photoreactivation after UVB reduced the apoptosis rate significantly, whereas empty liposomes had no effect. Likewise, photoreactivating treatment did not affect apoptosis induced by the ligand of CD95, CD95L. UVB exposure at 4°C, which prevents CD95 clustering, also reduced the apoptosis rate, but to a lesser extent. When cells were exposed to UVB at 4°C and treated with photolyase plus photoreactivating light, UVBinduced apoptosis was almost completely prevented. Inhibition of caspase-3, a downstream protease in the CD95 signaling pathway, blocked both CD95L and UVB-induced apoptosis, whereas blockage of caspase-8, the most proximal caspase, inhibited CD95L-mediated apoptosis completely, but UVBinduced apoptosis only partially. Although according to these data nuclear effects seem to be slightly more effective in mediating UVB-induced apoptosis than membrane events, both are necessary for the complete apoptotic response. Thus, this study shows that nuclear and membrane effects are not mutually exclusive and that both components contribute independently to a complete response to UVB.UV radiation in the middle-wavelength range between 290 and 320 nm (UVB) represents one of the most relevant environmental dangers because of its hazardous effects, including skin aging (1), induction of skin cancer (2), and exacerbation of infections (3). Like other adverse agents (alkylating chemicals, oxidants), UVB induces changes in mammalian cell gene expression (4-6). Elucidation of the underlying molecular mechanisms is of primary importance for the understanding of how UVB can damage cells and thus act as a pathogen. One of the most controversial issues in this context is whether the cellular UVB response is initiated at the cell membrane or in the nucleus (7). To exert its biological effects, UVB must be first absorbed by a cellular chromophore, which transfers the energy into a biochemical signal. Among a number of chromophores (porphyrins, aromatic amino acids, urocanic acid), DNA is regarded as the most important for several reasons. (i)The wavelength dependency of some UVB effects is similar to that for DNA absorption (8).(ii) Acceleration of DNA repair inhibits particular biological UVB effects (8-12). (iii) Lower UVB doses are necessary to achieve the same biological effects in DNA repair-deficient than -proficient cells (13). Thus, these data favor the concept that DNA is the most important mo...

Recent studies have described the role of shedding vesicles as physiological conveyers of intracellular components between neighboring cells. Here we report that melanosomes are one example of shedding vesicle cargo, but are processed by a previously unreported mechanism. Pigment globules were observed to be connected to the filopodia of melanocyte dendrites, which have previously been shown to be conduits for melanosomes. Pigment globules containing multiple melanosomes were released from various areas of the dendrites of normal human melanocytes derived from darkly pigmented skin. The globules were then captured by the microvilli of normal human keratinocytes, also derived from darkly pigmented skin, which incorporated them in a protease-activated receptor-2 (PAR-2)-dependent manner. After the pigment globules were ingested by the keratinocytes, the membrane that surrounded each melanosome cluster was gradually degraded, and the individual melanosomes then spread into the cytosol and were distributed primarily in the perinuclear area of each keratinocyte. These results suggest a melanosome transfer pathway wherein melanosomes are transferred from melanocytes to keratinocytes via the shedding vesicle system. This packaging system generates pigment globules containing multiple melanosomes in a unique manner.